About Ridge A

Antarctica: High and Dry (and Cold and Calm)

What makes a good observing site? Nearly every
kind of astronomical telescope benefits from a site that provides
clear, dry, and dark conditions. While the stormy
weather on the Antarctic coast is
legendary, the high plateau in the center of the
continent is quite the opposite. It lies at the eye of
the storm; it is a calm, clear
polar desert that offers
space-like observing conditions while comfortably
standing
on terra firma. Thus, siting a telescope at the
summit of the Antarctic plateau has significant
advantages over essentially anywhere else on Earth:

Characteristic

Ridge
A advantage

Clear

No clouds >80% of the time

High

>13,000 ft elevation for
100 miles

Dry

Super-dry polar desert (winter
PWV is 0.1 mm)

Cold

Winter temperature -100F,
mid-summer a balmy -30F.

Clean

Cleanest air on Earth, few
aerosols, little scattering

Dark

Almost 6 months of continuous
darkness

Good atmospheric seeing

~0.2 arcsec
above a ~15m ground layer

Stable climate

No prevailing
weather, seemingly constant

Minimal lightning

What's lightning?

Low wind throughout atmosphere

No
jet stream, calm surface winds (4 knots avg)

Low seismic activity

Seismically quiet

Accessible

Land a plane
right next to your telescope!

Continuous observing
possible

Most southern astronomical objects
are circumpolar

Relief map of Antarctica, showing the location
of Ridge A

No aerosols make for
nearly zero scattering. Would you believe that the Sun
lies behind that finger? Credit: Geoff
Sims

Late January summer temperatures of -42C
and only 55% of the atmosphere at sea level make
for a challenging work environment.

MPG video of 1 month of all-sky video thumbnails, showing the
benign, almost constant conditions at Ridge A, from 23 January to
23 February 2012.

In particular, one's ability to do infrared,
submillimeter-wave and terahertz astronomy
hinges on the
amount of water vapor in the atmosphere, since water
molecules absorb this light very effectively before it
reaches the ground.

Really! In the summer, there is enough moisture in the air at
most midlatitude locations on Earth, at the frequency of
the ionized
carbon line (1900 GHz), light can only travel 10-50 meters before it is
mostly absorbed by the intervening water vapor.

This effectively forces such
astronomical observatories to the highest and driest
sites where this atmospheric absorption is minimized.
The bitterly cold air holds no water vapor, and what
little remains freezes out into tiny ice crystals. This
makes the summit of the Antarctic plateau the driest
place on Earth.

Atmospheric transmission (fraction of light reaching the telescope
from space) through the terahertz portion of the
electromagnetic spectrum. Ridge A has the highest mean transparency of
any established site on Earth, opening entirely new frequency
windows to observation from the ground!

We selected the Ridge A site from satellite data
to be the best location for an astronomical observatory
on the Antarctic plateau, and indeed, anywhere on Earth.
It is located on the summit ridge of the
ice plateau at 81:40:25 South latitude and 72:42:58 East
longitude at a physical elevation of 13,260' (4040 m)
with a typical pressure altitude of 15,200'
(4650 m). It constitutes the origin of the continent's
famous katabatic winds and is perhaps
the calmest place on Earth, with typical winds of 4
knots (2 m/s). Even more important for infrared and
terahertz telescopes is the extreme cold. Winter
temperatures routinely drop below -100F (-70C), providing for
a very dry, stable, clear atmosphere. The extremely low amount of
water vapor that results allows
observations to be routinely performed here that cannot be done
reliably anywhere else on Earth. While it is perhaps the most
remote site on the planet, it is nevertheless still accessible by
aircraft or ground traverse.